It is well established that a glass article may be strengthed by uniform development of compressive stresses within a surface layer on the article. A technique for developing such stresses, known as chemical strengthening, involves exchange of ions between the glass and an external source.
One method of chemical strengthening by ion exchange is termed a low temperature method because it is normally carried out below the glass strain point. At such temperatures, the glass structure does not rearrange to any substantial degree, and therefore does not release stress to any substantial degree. In this method, relatively large ions, such as potassium or sodium, migrate into a glass and exchange position with smaller ions in the glass, such as sodium or lithium ions. The physical crowding caused by such large-for-small ion exchange creates compressive stresses. These are essentially retained since the glass structure cannot rearrange.
The low temperature method is described in detail in an article entitled: "Strengthening by Ion Exchange", Journal of the American Ceramic Society, pp. 215-219 (1964). The article explains that aluminosilicate or zirconosilicate glasses, that is silicate glasses containing substantial contents of Al.sub.2 O.sub.3 and/or ZrO.sub.2, are uniquely susceptible to such ion exchange. Substantial strength may be retained even after abrasion of these glasses.
The low temperature method is capable of generating relatively large strength values. However, the depth of the exchange, and consequent stress development, tends to be shallow. This can be corrected by longer exchange time, but this expedient also creates a relatively high central tension. That is the force developed in a central zone of the article to balance the compressive forces developed in the surface layer.
A method of coping with this problem is disclosed in U.S. Pat. No. 3,751,238 (Grego et al.). The depth of ion exchange below a glass surface within a given time is substantially increased, while maintaining a relatively low level of central tension in the article. The method comprises (1) contacting the surface of a sodium silicate glass article with potassium ions to create a potassium enriched surface layer by exchange of potassium ions for sodium ions, (2) heating the article at a temperature above the glass strain point for at least five minutes to cause molecular rearrangement in the glass to accommodate the potassium ions and release any compressive stress, and (3) contacting the glass surface with potassium ions at a temperature over 200.degree. C., but below the glass strain point. Ion exchange occurs at two levels. This results in a deeper level of ion exchange being attained while maintaining a moderate central tension and adequately high surface strength. Preferably, the first contacting and the heating steps are combined and may extend for a period of hours.
For many purposes, the improvements provided in accordance with the Grego et al. method were quite satisfactory. However, difficulty was encountered in attempting to strengthen sheet glass for vehicle windows. For this purpose, certain rather severe conditions are prescribed.
For test purposes, 6" or 12" squares are commonly employed for fracture pattern study and/or stone damage. However, the actual size of a vehicle side window is 12".times.36". We found this could make a critical difference. Thus, a test piece might appear to provide properties within a specification, whereas the full size product would fail.
Specifications not only vary from country to country, but also may undergo intrinsic change. For example, a recent European specification prescribed that 5 out of 6 samples must pass a ball drop test in which a 227 gram ball is dropped on a sample from a height of two meters. In a further test, a window is broken by impact with a 75 gram hammer having a point radius of 0.2 mm, and examined after a ten minute delay. The number of shards in a 6 cm square area (outside a defined impact area) should be between 40 and 350 in number with none larger than 3 cm.sup.2. At the same time, a United States specification has prescribed that 10 of 12 samples, each one foot square, must survive a half pound ball being dropped from ten feet. While shard size was not prescribed, no fragment more than 0.15 oz. was permitted after breakage by ball drop. Also, survival of impact by an 11 lb. sand bag from 8 feet was required.
To attain these levels, a depth of layer over ten mils, and preferably over 15 mils, was considered necessary; also a low central tension of no more than 5 kg/mm.sup.2, and preferably 3-4, with a surface compression of at least 30,000 psi.